PLAT/i2c: Fix the i2c device node name and add target property

Fix the i2c device node name that should show the functionality and
use a dedicated 'target' name property to tell targets being controlled.

Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>